Processes for the production of polymeric foams by reactive chemical routes are varied and well known. An example is flexible polyurethane foam which is produced in blocks typically 2 meters.times.2 meters.times.1 meter. These large blocks can be produced either continuously on conveyor type machines, or discontinuously in molds.
In the case of flexible polyurethane foam, the reacting mass achieves a high exotherm temperature within a very short time, typically between 5 minutes and 30 minutes. Blocks once made therefore have to be transferred to an intermediate "cure area" where they are carefully stacked with air space around each block until they have cooled. A large area is required for this purpose and the blocks typically need to be stored for a minimum of 10 hours before they can be restacked or loaded for transporting to the customer. This process of intermediate storage, to ensure adequate cooling of the blocks, is inconvenient and costly in space requirements. Further, the intermediate storage area contains a large number of blocks of inflammable foam at high temperature, presenting a potential fire hazard. The building used for this intermediate storage needs to be specially constructed to meet fire regulations.
A further important factor is that certain of the foam forming reactions are reversible at high temperature, typically the allophanate reaction following the initial polyurethane bond formation. The blocks of foam in the intermediate storage area are frequently at an internal temperature exceeding 140.degree. C. for several hours. If the ambient atmosphere in the intermediate area is not controlled, i.e. is of variable humidity, there is a potential for ingress of moisture into the block which will react with free isocyanate end groups and terminate them: EQU RNCO+H.sub.2 O.fwdarw.RNH.sub.2 +CO.sub.2
This reaction, removing the isocyanate required for the allophanate reaction, results in a reduced, uncontrolled level of cross linking in the foam and therefore a variable, reduced, stiffness or compression hardness. In geographical locations where a high ambient humidity is common, it is known for foamers to increase the quantity of isocyanate in a given foam recipe deliberately, in spite of the cost penalty in doing so, so as to allow for the hardness loss that would otherwise be experienced.
It has been proposed in PCT/GB85/00605 (published No. WO 86/04017) to use a new approach of early cooling, and specifically a method of making blocks of polyurethane or other foam arising from exothermic reaction of foam-forming materials, wherein once the reaction has reached a desired stage of completion a gas of suitable composition and temperature is passed through the body of the block to carry away the heat of reaction. Other earlier proposals are those of Riccardi et al U.S. Pat. No. 3,890,414 (published 1975) and Continental Gummi Werke German OLS 2,456,421 (published 1976). The cooling gas as proposed will normally be air, and the approach is the reverse of the conventional approach of slow cooling and minimum exposure to air while cooling takes place.
The conventional approach has to be seen in the light of a long standing problem in the polyurethane foam industry, namely autoignition of foam blocks due to excessive chemical exotherm. The problem occurs particularly with certain low density and high exotherm grades of foam, or foams containing additives which are included to render the foam resistant to small sources of ignition. Such foams can, after a period of two to three hours, and after they have started to cool, begin to increase in temperature again. This second exotherm is normally a self progressive type, eventually resulting in autoignition. Several factories have been burned down because of this phenomenon. One mechanism is thought to be the drawing in of air from the atmosphere as the block cools. The oxygen enriched atmosphere within the block then causes exothermic oxidation of the polyurethane polymer with a resulting temperature rise. The presence of air drafts around the blocks has been shown to exacerbate the problem.
The new approach in contrast deliberately supplies air, usually controlling its composition at least as to moisture content, but on its own has been found insufficient.
In particular, polyurethane foam commonly contains butylated hydroxy-toluene ("BHT" full name 2,6-ditertiarybutyl-4-methyl phenol), which is used as an antioxidant in the polyols that are reacted with isocyanates such as toluene diisocyanate ("TDI") to form the foam. This is a solid subliming at 70.degree. C. and therefore taken up in cooling air passed through blocks at initial temperatures of 140.degree. C. or higher. For heat recovery and control of moisture content, and also to prevent uncontrolled levels of residual isocyanate or auxiliary blowing agents such as chlorofluorocarbons ("CFCs") reaching the atmosphere, the cooling air is desirably recycled. This is done via a heat exchanger, and that then rapidly blocks up with a solid deposit of BHT together with some polyurea formed from residual TDI and moisture. Other antioxidants and additives cause similar problems.